1. Field of the Invention
Embodiments of the present invention generally relate to substrate supports used in processing chambers for fabricating microelectronic devices and, more specifically, to electrostatic chucks used in plasma processing chambers.
2. Description of the Related Art
Electrostatic chucks are widely used to hold substrates, such as semiconductor wafers, during substrate processing in processing chambers used for various applications, such as physical vapor deposition, etching, or chemical vapor deposition. Electrostatic chucks typically include one or more electrodes embedded within a unitary chuck body which comprises a dielectric or semi-conductive ceramic material across which an electrostatic clamping field can be generated. Semi-conductive ceramic materials, such as aluminum nitride, boron nitride, or aluminum oxide doped with a metal oxide, for example, may be used to enable Johnsen-Rahbek or non-Coulombic electrostatic clamping fields to be generated.
In a monopolar electrode chuck, the chuck comprises a single electrode which is electrically biased with respect to the substrate by an applied voltage. A plasma is introduced into the processing chamber to induce opposing electrostatic charge in the chuck and substrate to create an attractive electrostatic force that electrostatically holds the substrate to the chuck. In a bipolar electrode chuck, the chuck comprises two electrodes which are electrically biased relative to one another to provide an electrostatic force that holds the substrate to the chuck. Unlike the monopolar electrode chuck, the bipolar chuck does not require the presence of a plasma to generate an electrostatic clamping force.
Electrostatic chucks offer several advantages over mechanical clamping devices and vacuum chucks. For example, electrostatic chucks reduce stress-induced cracks caused by mechanical clamping, allow larger areas of the substrate to be exposed for processing (little or no edge exclusion), and can be used in low pressure or high vacuum environments. Additionally, the electrostatic chuck can hold the substrate more uniformly to the chucking surface to allow a greater degree of control over substrate temperature. This control may be further enhanced by using a heat transfer gas for thermal coupling between the chuck and substrate.
Various processes used in the fabrication of integrated circuits may require high temperatures and wide temperature ranges for substrate processing. Such temperatures may range from about 20° C. to about 150° C., and possibly as high as 300° C. to 500° C. or higher for some processes. It is therefore often desirable to have an electrostatic chuck which can operate over a wide range of temperatures.
To utilize the advantages of an electrostatic chuck, the electrostatic chuck typically forms part of a substrate support assembly which also includes various components for heating and cooling the substrate and for routing power to the chuck electrodes. In addition, the substrate support assembly may also include components for providing a substrate bias and for providing plasma power. As a result, the ceramic body of the electrostatic chuck may include additional electrodes and other components, such as heating elements, gas channels, and coolant channels, to name a few. Also, the electrostatic chuck may be attached to supporting components which are made of metal.
However, it is difficult to attach metal components to or embed metal components (e.g., electrodes) within the ceramic chuck body because of differences in the thermal expansion coefficients (CTE) of the ceramic and metal which can result in thermo-mechanical stresses that can cause the ceramic to fracture or chip during thermal cycling. Additionally, differences in the CTEs may increase with temperature resulting in greater thermo-mechanical stresses at higher temperatures. To compensate for these stresses, the ceramic chuck body may be made thicker to provide greater strength and prevent fracture, but this often adds cost to the chuck body.
Additionally, gas conduits and electrical lines are often coupled to the electrostatic chuck through interfaces or feedthroughs which provide vacuum seals. The feedthroughs may be sealed by polymer o-rings. However, the polymer o-rings often lose compliance and resilience at high temperatures which can lead to failure of the vacuum seal. Also, any fracture of the ceramic chuck due to thermo-mechanical stresses caused by differences in the CTEs can result in seal failures and vacuum leaks.
In certain applications, it may be desirable to apply a bias to the substrate and/or generate a plasma by coupling radio frequency (RF) power at the electrodes of the electrostatic chuck. The efficiency of the RF power transmission is dependent in part on various properties of the chuck body, such as the thickness and dielectric constant of the dielectric layer between the electrodes and substrate. For applications where RF power may be applied over a wide range of frequencies, such as between about 50 kHz and about 60 MHz, for example, it may be desirable to have an electrostatic chuck which can be optimized in a cost effective way for efficient RF power transmission over a wide frequency range.
Therefore, a need exists for a cost effective electrostatic chuck that can operate at high temperatures and over a wide range of temperatures in a high vacuum environment without failure. Additionally, a need exists for a cost effective electrostatic chuck which can efficiently couple RF power over a wide frequency range.